simple-stream: unnest memory_input_stream::simple and fragmented
simple-stream: unnest memory_input_stream::simple and fragmented

Nested classes cannot be forward-declared which complicates proper
ordering of declarations and definitions.

cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar